Update on the risks of complications after knee arthroscopy

Knee arthroscopy is one of the most common surgical procedures worldwide [1] and the number of arthroscopies has substantially increased in the last 30 years [2]. The patients are often younger or middle-aged physically active individuals. Annually, there are about one million such procedures performed in the United States and in Sweden (population 9.5 million) the corresponding number is about 35,000 [2, 3]. The most common procedures are meniscectomy, meniscal repair, and cruciate ligament reconstruction [2, 4, 5].

Knee arthroscopy is widely acknowledged to be a safe procedure [4, 6]. Still, there are also known serious complications such as joint infection, deep vein thrombosis, pulmonary embolism, and there are even deaths reported [5, 7, 8]. For more complex arthroscopic procedures that involve ligament reconstruction, the risk of complication has been reported to be the highest followed by meniscal repair, chondroplasty, and meniscectomy [5, 7]. However, there is a substantial variation in the reported absolute risks for complications – pyogenic arthritis between 0.08% [4] and 0.42% [9], deep vein thrombosis between 0.12% [5] and 41% [10], and pulmonary embolism between 0.03% [11] and 0.11% [7].

The absolute risk of having a complication such as pyogenic arthritis, VTE, or other surgical complications after knee arthroscopy was 1.1%. The increase in risk of these conditions as compared to the general population was about 9-fold, and compared to other patients with a knee condition (but no knee arthroscopy) about 5-fold.

There is a need for updated estimates on the risks associated with knee arthroscopy as there has been an increase in number of procedures, but the procedure is also more and more frequently performed also in middle-aged patients. Also the surgical routines may have improved which calls for a need of new estimates of risks associated with the procedure. Thus, we used comprehensive Swedish health care data over a 12-year time period to evaluate the risks for the most frequent complications during a 30-day follow-up period.

It has been previously reported that knee arthroscopy is associated with complications such as pyogenic arthritis and VTE [4, 7, 10, 17]. However, the estimated over-all risk of complications varies from 0.27 to 8.2% with the risk being higher after more complex and longer lasting arthroscopies [7]. Reigstad et al. [8] presented an over-all risk of 5%, but pain and swelling were included in this study which represented almost 50% of all complications. In our study, we focused on the more severe complications which explain why our estimate is lower. Sherman et al. [6] presented the over-all risk of complications after knee arthroscopy to be about 8%. In the study the authors classified complications into minor and major. Several of the major complications were more similar to the ones selected in our study such as infections, cardiovascular, neurological and instrument failure. Further, the relatively high over all risk could also potentially partly be explained by the study being 20 years old and that arthroscopic surgery has developed since with better equipment and better surgical routines at large. In the rest of the existing literature, which is somewhat more recent, the reported risk of complications is reported to be below 5%. In one study from Japan [4] the investigators report a very low over all risk for complications of merely 0.27% in a relatively small cohort. In larger cohort studies the risk is typically reported to be between 0.64 and 4.7% [7, 18, 19], which is more in line with our findings. The follow-up period in most studies has typically been up to some 30 days [5, 7, 9, 18, 19] with the exception of a few studies with 90 days of follow-up [8, 11, 18]. In our study, we chose 30 days of follow-up because the most common complications, such as infections and VTE, attributed to the surgery are likely to be diagnosed within that time frame.

Partial meniscectomy was the most common procedure in our study (44% of all knee arthroscopies) which is in line with existing literature [2, 5, 8, 11]. Although arthroscopic partial meniscectomy carries a smaller risk for complications as compared to more complex surgery, such as ligament reconstruction, the reported overall risk is still not negligible [7]. This is in line with our results, where even after excluding the ligament reconstruction from the arthroscopy group, the relative risk of a complication was still almost 7-fold as compared to the general population.

Since knee arthroscopy is the most common orthopaedic procedure this results quite many patients experiencing a complication even though the absolute risk is low. For example, our data suggests that 5% of all cases of pyogenic arthritis are caused by knee arthroscopy. Thus, indications for arthroscopic surgery need to be scrutinized as proper evidence of efficacy above non-surgical management is sometimes lacking and compliance to new guidelines still needs to be verified, e.g. for degenerative meniscal lesions and knee osteoarthritis [20, 21].

Our study is based on a large study population from an entire Swedish region without selection bias. The original data is all prospectively registered in the electronic medical records, i.e., free of any potential recall bias from both the patient and doctor. We estimated not only the absolute risk for complications after knee arthroscopy but also the relative risk compared to the background population. Still, we would like to acknowledge some important limitations: As in all register-based research, there is a risk of misclassification of procedures and diagnostic codes which may lead to biased results. However, the validity of the diagnostic codes in SHR and Swedish health care registers in general has been reported to be high [22‐24]. We would like to emphasize that these registers are not just administrative data per se, as in many other countries, but diagnoses are actually set by the doctors’ themselves and data is drawn directly from the patients’ electronic medical records. Further, we only included complications diagnosed within specialist care, where the validity of the diagnostic codes (such as pyogenic arthritis or VTE) is expected to be higher than in primary care. We did not include the complications registered on day 0 to minimize the risk of reverse causality (pyogenic arthritis). Thus, the true absolute risks may rather have been underestimated, but is not expected to substantially influence the relative risks or the population attributable fraction. Another important limitation is a potential for bias from unmeasured confounding (e.g. smoking and body mass index) and bias by indication – i.e. the systematic differences between persons undergoing arthroscopic surgery and the underlying population. We aimed to take the latter into account in our sensitivity analysis, where only persons with a potential indication for a knee arthroscopy were included which yielded similar results, except for pyogenic arthritis where we noted a substantial attenuation of the risk estimate. Unfortunately, there is no data on body length and body weight available in the register.

To conclude, although the absolute risk of complication of 1.1% is small, knee arthroscopy should not be considered a completely benign intervention. In particular as it is high volume surgery at most knee surgery clinics. For example, the procedure may be responsible for 5% of cases of pyogenic arthritis of the knee. Thus, it remains important to communicate risks with the patient, and consider non-surgical treatment options when appropriate.